fix: Allow only one releasable column per unique index (#91)

test/serverpod_offline_sync_test_server/lib/src/demo_sync_tables.dart

@@ -24,6 +24,7 @@ final demoSyncTables = <Table>[
   Types.t,
   Unique.t,
   UniqueComposite.t,
+  UniqueDiscriminator.t,
   UniqueSetNullChild.t,
   UniqueUuid.t,
 ];

test/serverpod_offline_sync_test_server/lib/src/endpoints/demo_debug_endpoint.dart

@@ -295,6 +295,7 @@ class DemoDebugEndpoint extends Endpoint {
     await add<Unique>();
     await add<UniqueUuid>();
     await add<UniqueComposite>();
+    await add<UniqueDiscriminator>();
     await add<UniqueSetNullChild>();
     await add<Types>();
     await add<FkChainRoot>();

test/serverpod_offline_sync_test_server/lib/src/models/unique_discriminator.spy.yaml

@@ -0,0 +1,9 @@
+class: UniqueDiscriminator
+table: unique_discriminator
+database: all
+fields:
+  id: UuidValue?, defaultPersist=random_v7
+  ### Owner scope of this row. Maintained by the CRDT sync layer.
+  scopeId: int?, relation(parent=crdt_scopes, onDelete=Cascade)
+  categoryId: int
+  name: String, unique(per=[scopeId, categoryId])

test/serverpod_offline_sync_test_server/lib/src/models/unique_no_release.spy.yaml

@@ -0,0 +1,8 @@
+class: UniqueNoRelease
+table: unique_no_release
+database: all
+fields:
+  id: UuidValue?, defaultPersist=random_v7
+  ### Owner scope of this row. Maintained by the CRDT sync layer.
+  scopeId: int?, relation(parent=crdt_scopes, onDelete=Cascade)
+  categoryId: int, unique(per=[scopeId])